The purpose of this study is to explore the significant roles of cardiac autonomic control in sleep-disordered breathing (SDB) during various sleep stages in an established cohort of 700 children randomly generated from public elementary schools (K-5). The cohort was originally established from an NHLBI-funded R01 study entitled "Prevalence of Sleep-Disordered Breathing in Children (R01-HL-63772-05)." The objectives of the original study were to evaluate the prevalence of childhood SDB and to identify correlates. One of the major findings was that systolic blood pressure was significantly associated with SDB independent of BMI. The baseline examinations included a standardized overnight sleep laboratory study, comprehensive sleep history, physical examination and a psychometric battery that assesses psychological and cognitive functions. Extensive data on SDB, episodes of apnea, sleep quality, and sleep stages were collected. As a "by-product" of the baseline standard 9-hour polysomnogram, a single channel of continuous EKG data were also recorded and stored. We propose to process and analyze these EKG data, which is a new task not included in the original study. We will derive time and frequency domain measures of heart rate variability (HRV) indices as measures of cardiac autonomic control. Specifically, we will calculate overall 9-hour HRV, sleep stage specific 5-minute HRV, and before and after apnea episode 5-minute HRV. The HRV indices will be analyzed together with the extensive and already available SDB data to examine the following research questions: (1) Whether SDB is associated with poor HRV profiles, indicating impairment of cardiac autonomic control; (2) Whether there is a dose response relationship between the degree of SDB and HRV impairment; (3) Whether there is an effect modification by BMI on the above SDB and HRV associations; (4) Whether impaired autonomic control as measured by HRV explains the relationship between SDB and elevated blood pressure in children; (5) Whether an episode of sleep apnea leads to immediate impairment of cardiac autonomic balance; and (6) What is the quantitative relationship between sleep stages (REM, nonREM, and wake stages) and cardiac autonomic control in children, and whether SDB has a negative impact on such relationship? This proposed cross-sectional study is based on our previous extensive works on cardiac autonomic control, SDB in children and adults, and metabolic disorders. It is highly responsive to a recent RFA (Program Announcement Number: PA-06-238, "Research on Sleep and Sleep Disorders (R21)." Analysis of these cardiac autonomic control data in combination with extensive sleep data from the original study will enable us to determine whether we can successfully extend our traditional sleep disorder study in cardiac risk assessment, which has grown into a distinctive and matured research and application in cardiology. This project will also enable us to test a novel research hypothesis - that SDB is associated with higher cardiovascular disease risk partially by its negative impact on cardiac autonomic control towards parasympathetic impairment and sympathetic over-activation. About 20-28% school-aged children have SDB, and 1-3% school-aged children have obstructive sleep apnea, the most severe form of SDB. This project will enable us to successfully extend our traditional sleep disorder study into cardiac risk assessment. This project will also enable us to test a novel research hypothesis - that SDB in children is associated with higher cardiac risk partially by its negative impact on cardiac autonomic control towards parasympathetic impairment and sympathetic over-activation, which can lead to decreased threshold for fatal arrhythmias and elevated blood pressure. The knowledge gained from this study can potentially improve the management and treatment of childhood SDB. [unreadable] [unreadable] [unreadable] [unreadable]